diff --git a/src/internal/headers.ts b/src/internal/headers.ts
index c724a9d22..4894a378d 100644
--- a/src/internal/headers.ts
+++ b/src/internal/headers.ts
@@ -74,7 +74,7 @@ export const buildHeaders = (newHeaders: HeadersLike[]): NullableHeaders => {
   for (const headers of newHeaders) {
     const seenHeaders = new Set<string>();
     for (const [name, value] of iterateHeaders(headers)) {
-      const lowerName = name.toLowerCase();
+      const lowerName = name.toLocaleLowerCase('en-US');
       if (!seenHeaders.has(lowerName)) {
         targetHeaders.delete(name);
         seenHeaders.add(lowerName);
diff --git a/tests/buildHeaders.test.ts b/tests/buildHeaders.test.ts
index 3f8e4d28e..2af2bf48a 100644
--- a/tests/buildHeaders.test.ts
+++ b/tests/buildHeaders.test.ts
@@ -79,6 +79,25 @@ describe('buildHeaders', () => {
     ],
     [[undefined], `NullableHeaders { }`],
     [[null], `NullableHeaders { }`],
+    [
+      [
+        {
+          'OpenAI-Organization': 'org-123',
+        },
+      ],
+      `NullableHeaders { 'OpenAI-Organization': 'org-123' }`,
+    ],
+    [
+      [
+        {
+          'OpenAI-Organization': 'org-123',
+        },
+        {
+          'openai-organization': 'org-456',
+        },
+      ],
+      `NullableHeaders { 'openai-organization': 'org-456' }`,
+    ],
   ];
   for (const [input, expected] of cases) {
     test(expected, () => {